This invention relates to a packet communication system of a multiaccess type wherein a single communication medium is used in common by a plurality of users for effecting packet communication.
A TDMA (time division multiple access) system and an ALOHA system have been well known as the packet communication system which uses a satellite or the like as a single communication medium as shown in FIG. 1 of the accompanying drawing. According to the former system, time is divided into a plurality of time slots, each for transmission of one packet. Thus, where there are N (a positive integer) users, N time slots are made to correspond to one frame and each time slot in one frame is fixedly assigned to each user. This system is advantageous when the number of users is small and the traffic of each user is large, however when the number of users is large and the traffic of each user is small, the utilization efficiency of the channel decreases greatly, thus greatly increasing the average delay time. Furthermore, this system is not advantageous where the traffic among users is not uniform. The latter ALOHA system includes a pure ALOHA system developed several years ago and a slotted ALOHA system developed later. The invention will be described in connection with the slot ALOHA system. As shown in FIG. 2, time is divided into a plurality of time slots in the same manner as the TDMA system. When a user wants to transmit a packet, he immediately transmits the packet in synchronism with a given time slot. If a number of users transmit packets causing collision of packets at the given time slot, packets are retransmitted at random. This system is advantageous for a case where the number of users is large and the traffic of respective users is small. However, the maximum throughput of this system is as low as 0.368 with respect to the optimum channel capacity defined as 1.0. Accordingly, where the traffic exceeds this value, the number of collisions increases, thus causing a deadlocked state.
Accordingly, when the number of users is large and the traffic varies in a wide range, neither system described above may be suitable. One option is to selectively use either one of the ALOHA mode or the TDMA mode, according to the traffic. Such option is shown in U.S. patent application Ser. No. 503,264 by the same applicant as the present application entitled "Multiple Access System and Method" and filed on June 10, 1983. When a system of this U.S. patent application is applied in a time domain, it assumes the ALOHA mode when the traffic is small or the TDMA mode when the traffic is large. As shown in FIG. 3, according to this system, a time interval is divided into a plurality of time slots for respective transmission times of one packet so that M slots are contained in one frame and the users would be assigned through parameters that correspond to respective time slots in one frame.
Let us denote a parameter of a user j corresponding to a time slot i by P.sub.ij, then, when the user j desires transmission inclusive of retransmission at the time slot i, the transmission is made at a probability of P.sub.ij. If the transmitted packet collides with another packet in the channel, the transmitted packet is delayed in the same manner as the slotted ALOHA system according to a certain distribution for retransmission.
The following is an algorithm for adjusting P.sub.ij, the basic form thereof being shown as follows: ##EQU1## In the above and T represents time the unit of which is one frame (M slots), .alpha. represents a positive small number of less than 1 (one) which is called a correction coefficient. Since a parameter {P.sub.ij } represents the probability of transmission, the following restriction is assumed in addition to the algorithm described above: ##EQU2##
A low traffic condition will first be considered. Under this condition, the channel is empty in most cases so that the correction is made in a direction for increasing the parameter P.sub.ij with the result that P.sub.ij becomes 1 under the conditions (3). Under these conditions, an operation identical to that of the conventional slotted ALOHA system can be made because an immediate transmission can be made in response to a transmission request. This state is hereinafter termed an ALOHA mode.
A case of a high traffic will now be considered. In the initial ALOHA mode, all parameters {P.sub.ij } are once reduced owing to frequent collisions. When the parameters become sufficiently small, a successful packet will appear. Suppose now that a packet sent out to a time slot i from user j is successful, then parameter P.sub.ij increases whereas the parameters P.sub.ik (k.noteq.j) of other users decrease. Accordingly, the user j has a good chance for transmitting the packet at the time slot i, thus increasing the probability of success of the transmitted packet. As a consequence, the parameter P.sub.ij of user j continues to increase until P.sub.ij =1, while the parameters P.sub.ik (k.noteq.1) of other users continue to decrease until P.sub.ik =0 (k.noteq.j). When the parameters are determined in this manner, no collision occurs at the time slot i. The parameters of other time slots are determined in the same manner, with the result that a system in which collision is avoided can be realized.
Although the prior art system described above is extremely efficient for a data transmission in which packets arrive in accordance with Poisson distribution and differences in delay are permissible, there arises a problem for voice packets which arrive periodically and requires immediate transmission, because the delay is varried due to the traffic.